A high speed photographic printer typically includes a light source under which a roll of developed film containing images on negatives is rapidly and continuously passed for reproducing the images on a roll of photosensitive paper, which is placed beneath the film. A lens assembly is placed between the paper and the roll of film for focusing the image and the amount of light which is directed onto the paper. The lens assembly includes two moveable groups of elements for controlling the magnification of the image and an iris diaphragm for controlling the amount of light.
The typical iris diaphragm includes a plurality of moveable blades for forming an adjustable aperture through which the light passes. The blades are typically controlled by a single stepper motor which, via movement of the blades, induces the variation of the aperture. This configuration is discussed in detail in U.S. Pat. No. 4,444,489. U.S. Pat. No. 4,381,146 discloses a standard iris diaphragm with an "inch worm" type ceramic motor.
Although the above described system and method for varying the aperture are satisfactory, they are not without drawbacks. It takes approximately two or three seconds to adjust the diameter of the aperture, and for high speed photographic printers, such a response time is insufficient to allow the aperture to change between images on the same roll of film because of the rapid speed at which the film is moving under the light source. High speed photographic printers typically require a quick response time, approximately 50 milli-seconds between images. Therefore, the entire roll of film is printed at one aperture setting, and if changes are needed for any of these printed images, the aperture is then adjusted and the copying process repeated. This consumes time, which obviously adds additional cost to the printing process.
In addition, the "inch worm" type motor requires multiple elements to operate and a relatively complex control scheme.
Consequently, a need exists for an iris diaphragm which includes a fast response time for permitting aperture variations between images, and which is economical to manufacture.